Commercially available sapphire fiber waveguides generally comprise thin cylindrical single crystal sapphire filaments without optical claddings. The wave-guiding structure is formed by the sapphire core and the surrounding air, which functions as an equivalent cladding. Due to the large refractive index difference between the sapphire fiber and the air, such waveguides are generally characterized by a very large numerical aperture (NA), a high degree of multimode transmission, and high sensitivity to bending. Moreover, the intensity of the optical signal propagating in the waveguide can be easily affected by changes in the surrounding atmosphere, such as refractive index variations and physical contacts, which are unavoidable in many applications. These problems make the fabrication of sapphire fiber sensors extremely difficult. For example, the highly multimodal nature of conventional sapphire fibers necessitates a precision of angular alignment of the sapphire fiber to an order of 0.1 degrees in sapphire fiber interferometers while communication-grade silica fiber-based interferometry requires a precision of only several degrees.
Cladding of an optical fiber is defined as: one or more layers of material of lower refractive index, in intimate contact with a core material of higher refractive index. Source: Federal Standard 1037C, Telecommunications: Glossary of Telecommunication Terms; Date of Publication: Aug. 7, 1996.
Materials and processes to produce suitable claddings on single crystal sapphire optical fiber waveguides are needed to reduce or eliminate all of the problems mentioned above. Single crystal sapphire fiber optic sensors and other sapphire waveguide structures require development of suitable cladding materials.